This invention concerns AlMgSi alloys of the 6000 series of the Aluminum Association Register. The compositions are low magnesium containing AlMgSi alloys with appropriate silicon and copper additions to meet the strength requirements of AA6063T5 and T6. AA6063 accounts for approximately 80% of all aluminium extruded products. At this bottom end of the extrusion market, there is a need for extrusions which meet the T5 or T6 strength requirements but which can be manufactured at the highest possible rates of extrusion.
This need was addressed in a paper by D Marchive in Light Metal Age, April 1983, pages 6-10. The author reported a trend towards reducing the content of Mg2Si and compensating for this by increasing the excess of Si, but that resulted in loss of formability. He reported new alloys in which the concentrations of Mg, Si, Cu, Mn and Cr were optimised to provide alloys which exhibited the required tensile properties but with superior extrudability, formability and toughness. The alloys had Mg contents in the range 0.35 to 0.60.
There has been a prejudice in the industry against reducing the Mg content of 6000 series general purpose extrusion alloys below 0.35 wt %. Of the 63 6000 series alloys listed in the Aluminum Association Register, all the general purpose extrusion alloys require a Mg content of at least 0.35 wt %.
WO 95/06759 describes high strength high extrudability AlMgSi Alloys having the composition in wt %: Mg 0.25-0.40; Si 0.60-0.90; Fe up to 0.35; Mn up to 0.35 preferably 0.10-0.25. But these are not general purpose extrusion alloys. By virtue of the high Si content they have high tensile strength generally in excess of 250 MPa and they preferably contain Mn to improve extrusion surface quality.
This invention concerns general purpose extrusion alloys having the minimum alloying additions required to meet the strength requirements of AA 6063T5 (peak aged tensile strength of at least about 152 MPa) and T6 (peak aged tensile strength of at least about 207 MPa). Decreasing the Mg content of such an alloy reduces the flow stress of the material at the temperatures used for extrusion, which in turn reduces the extrusion pressure and the work done in the process. Approximately 90% of the work of extrusion is converted to heat which results in temperature rise in the extruded product. With the dilute alloys described here, less heat is generated in the extrusion process as compared with conventional alloys, such that the product can be extruded at a higher speed before surface deterioration occurs. Usually the productivity of an aluminium extrusion is limited by the onset of various types of surface defect which is linked to the attainment of a critical temperature at the surface of the product.
The lower breakthrough pressure associated with the lower Mg content also means that for a given extrusion press, the initial billet temperature can be reduced until the pressure required matches the press capacity. This has the effect of further reducing the temperature of the product as it exits the die which gives further productivity benefits.
In one aspect the invention provides an alloy of composition in wt %
incidental impurities up to 0.05 each 0.15 total balance Al provided that when Mg is at least 0.30 and Cu is at least 0.05, then Fe is greater than 0.15.
The invention also provides extrusion ingots of the alloy as defined; and extrusions (i.e. extruded sections) made from such ingots.